OBJECTIVES: 1. The various mechanisms by which acetylstrophanthidin and ouabain increase the concentration of norepinephrine in the synaptic clefts of blood vessels. 2. The action of amitryptyline, a tricyclic antidepressant, on neurotransmitter release from sympathetic nerve terminals. 3. The action of lithium on the release of neurotransmitter from sympathetic nerve terminals. 4. The response of blood vessels to intense cold, in relation to the phenomenon of cold vasodilatation. 5. The different responses of forearm and calf blood vessels in man to reflex and emotional stimuli.